package/blktrace: do not build in parallel

package/blktrace: do not build in parallel

The Makefile is not parallel safe.

Fixes numerous build failures:
    http://autobuild.buildroot.org/results/c7f/c7f0b22c6d256c75ade223af702c5f29c6c677d8/
    http://autobuild.buildroot.org/results/4ab/4abb0d2590528c634766103ee8ffd648ad5c04d3/
    ...

Look for the message:
    btreplay.o: file not recognized: File truncated

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>